Solder is used as a material to bond an electronic part to a substrate in an electronic device, and use of lead-free solder is proceeding. However, in the case of the solder to be used for a bonding portion of a power device that reaches a high temperature during use, it is difficult to achieve lead-free solder. In recent years, development of a wide band gap semiconductor operable at higher temperatures than Si has been undertaken. The operating temperature of such power device is predicted to reach about 300° C. Thus, superior heat resistance is required of a bonding material.
Currently, sinter-bonding metallic particles are candidate of the high thermostability bonding material, that is cured by heat or electromagnetic waves. For example, there is suggested a paste prepared by covering metallic nano particles having high activity with an organic compound that is easily degraded by heat. In addition, there is suggested a method of making a high-strength joint using metallic nano particles. Further, there is suggested a method of forming a fine wiring pattern on a substrate using metallic nano particles.
A bonding portion formed by using the conventional metallic particle paste and subjecting such to a heat treatment at about 200 to 350° C. is a cured product consisting of porous metal having fine voids. The cured product is in a state in which the sintering process is stopped in the middle of the process. Thus, when the product is left to stand at the curing temperature or in high-temperature environments, the diffusion of metal in the bonding portion is progressed. Accordingly, fine voids aggregate near the interface between the bonding portion and a chip or near the interface between the bonding portion and an electrode, resulting in deterioration of the structure. This has been reported until now.
If attention is paid to the fact that the deterioration mechanism in high-temperature environments is based on the diffusion of metal, an effective method generally includes dispersing a small amount of a second element in the grain boundary phase of the metal structure and stabilizing the structure by a pinning effect. In order exhibit the pinning effect, the type of the second element is not particularly limited. It is important to uniformly disperse the second element in the grain boundary phase. In order to obtain a structure in which the second element is uniformly dispersed in the grain boundary phase using the metallic particle paste, a method of mixing fine particles of the first metal as base particles with fine particles of the second metal has been reported.
However, fine particles usually aggregate to form a secondary particle. Accordingly, it is very difficult to obtain a metallic particle paste containing the fine particles of the first metal and the fine particles of the second metal mixed uniformly.